Anaerobic Infections



  • Anaerobic bacteria are organisms capable of growing in a reduced oxygen environment, either exclusively (obligate anaerobes) or in addition to growing in air (facultative anaerobes).
  • Anaerobic bacteria can cause invasive and serious infections.
  • Anaerobic bacteria tend to participate with other anaerobic and aerobic flora in causing polymicrobial infections.


  • Although anaerobic bacteremia is less frequent in children than in adults, other anaerobic infections such as chronic sinusitis or chronic otitis media are common in children.
  • Because of their fastidious nature, the ability of microbiology laboratories to identify anaerobic bacteria is highly dependent on proper collection and transport of culture specimens; hence, anaerobic bacteria are often missed and likely underreported.

Risk Factors

  • Impaired host immunity
    • Malignancy
    • Splenic dysfunction
    • Hypogammaglobulinemia
  • Presence of devitalized tissue
  • Surgery, trauma
  • Vascular insufficiency
  • Poorly controlled diabetes
  • Presence of foreign bodies
  • Colitis


  • Anaerobic infections commonly derive from the normal flora of the oropharynx, skin, intestines, or the female genital tract; thus, anaerobic infections occur when there is a loss of integrity of anatomic or epithelial barriers at these sites.
  • Virulence factors include production of exotoxins (e.g., Clostridia spp.), endotoxins (e.g., Fusobacterium spp.), and presence of phagocyte-inhibiting capsules (e.g., Bacteroides spp.).


The most common clinically-relevant anaerobes include the following:

  • Gram-negative rods (Bacteroides, Prevotella, Porphyromonas, Fusobacterium)
  • Gram-positive cocci (Peptostreptococcus, Peptococcus)
  • Spore-forming gram-positive bacilli (Clostridia)
  • Non–spore-forming gram-positive bacilli (Eubacterium, Bifidobacterium, Propionibacterium, Actinomyces, Lactobacillus)
  • Gram-negative cocci (Veillonella, Acidaminococcus)
  • Spirochetes (many of which are anaerobic)

Commonly Associated Conditions

  • CNS infections:
    • Brain abscess due to bacteremia
    • Subdural empyema
    • Epidural abscess (most commonly due to complications from sinusitis)
  • Head and neck infections:
    • Sinusitis (generally polymicrobial)
    • Chronic otitis media
    • Ludwig angina (infection of the submandibular space)
    • Cervical adenitis
    • Peritonsillar abscess
    • Dental abscess
    • Gingivitis
    • Actinomycosis of jaw
    • Lemierre disease
      • Septic thrombophlebitis of the internal jugular vein owing to anaerobic bacteremia
      • Most commonly with Fusobacterium spp., often resulting in pulmonary abscess formation and metastatic infection
  • Pleuropulmonary infections:
    • Aspiration of infected amniotic or vaginal secretions in neonates
    • Aspiration of oral or gastrointestinal fluids in children (severe gingival or periodontal may be a risk factor)
    • Pneumonia, abscess formation due to aspirated foreign bodies
    • Actinomycosis
  • Peritonitis/peritoneal abscess:
    • Appendiceal abscess
    • Perforated viscus
    • Postoperative complication
    • Trauma-related
    • Actinomycosis
  • Cholangitis
    • Ascending infection may occur following biliary tract surgery (e.g., Kasai procedure).
  • Soft tissue infections:
    • Paronychia
    • Pilonidal cyst
    • Hidradenitis suppurativa
    • Crepitant cellulitis
    • Necrotizing fasciitis
    • Gas gangrene (Clostridium spp.)
    • Infected decubitus ulcers (may result in contiguous osteomyelitis)
    • Penetrating wounds (may lead to tetanus)
  • Infections of the female genital tract:
    • Endometritis
    • Salpingitis
    • Tubo-ovarian or adnexal abscess
    • Pelvic inflammatory diseases
    • Pelvic abscess
    • Bartholin gland, vulvar, or perineal abscess
    • Bacterial vaginosis
  • Infected bite wounds
    • Anaerobes isolated from 50% of human or animal bites
  • Bacteremia
    • Often associated with focal primary site of involvement (gastrointestinal disease, abscess)
  • Neonatal infections:
    • Cellulitis at fetal monitoring sites
    • Aspiration pneumonia
    • Omphalitis
    • Conjunctivitis
    • Infant botulism

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